Hi, I'm Nicholas Johnson!

software engineer / trainer / AI enthusiast

Associations Between Models

We’re going to add Comments to our blog. Comments belong to BlogPosts and blog_posts have many comments.


First of all, scaffold the Comment model. Refer to the last exercise if you can’t remember how to do this.

Comments will need several fields, I’ll leave this part up to you, but crucially, comments will need a blog_post_id: integer field. Notice how I highlighted that part.

Now set up the relationships

You’ll need to extend your models something like the following:

  class BlogPost
    has_many :comments

  class Comment
    belongs_to :blog_post

Test the association

Drop into the console and check your association. You should be able to call something like:

  post = BlogPost.first


Add validation to your comment. A comment needs a bog_post_id to be valid, plus a couple of other fields. Enforce this.

Listing comments

On your blog_post show page, list all the comments for a particular blog post. Remember you can use @blog_post.comments to get an array of the comments.

Great. You can now create comments from the comments form, and see them when you view a blog but you will need to manually enter the blog_post_id when creating the comment. Let’s fix that.

Extension: Nested Routes

Use a nested route to nest your comment inside a BlogPost. Modify your routes.rb file like so:

  resources :blog_posts do
    resources :comments

Use rake routes to check the routes you have made.

  rake routes

=code(code, :bash)

Now you can visit a URL like

http://localhost:3000/blog_post/1/comments/new to create a comment. Remove the blog_post_id field from the comment form view. Instead, in your controller set it using the params hash. You may need to drop into the debugger to check the params hash here.

Optional finishing up

Pick from the following:

  • See if you can integrate your comment form right into your blog_post_show page. (tip, in the blog_post_show controller execute:
  @comment = Comment.new.

This will let the comment form just work without modifications.)